In recent years, in hygiene products such as disposable diapers, sanitary napkins and incontinent pads, the water-absorbing resin as a composition material thereof has been widely used, as a water-absorbent agent, in view of absorbing a body liquid. As such a water-absorbing resin, for example, there have been known a cross-linked product of partially neutralized polyacrylic acid, a hydrolysate of a starch-acrylic acid graft polymer, a saponified product of a vinyl acetate-acrylate ester copolymer, a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer and a cross-linked product thereof, and a cross-linked product of a cationic monomer and the like. This water-absorbing resin may be used after converted to also a sheet-like, fiber-like or film-like form, however, it is generally used in a water-absorbing agent after converted to a powder-like form (particulate). As such powders (particles), for example, the particulate water-absorbing agent, having a weight average particle diameter thereof of about 200 to 800 μm, is used widely.
The particulate water-absorbing agent is produced via many steps (preferably continuous steps) (for example, Patent Document 1). When the particulate water-absorbing agent is shipped, this particulate water-absorbing agent is filled in a member for filling. The particulate water-absorbing agent filled is transported by a transportation means and delivered to a shipping address (a user or the like). As this member for filling, for example, a flexible container bag is used. This flexible container bag is also referred to as “FLECON bag” in Japanese in abbreviation. This filling is usually performed from a hopper as described in Patent Document 2 or the like.
The particulate water-absorbing agent has been produced under control of various parameter properties (for example, absorbency, absorbency against pressure, water absorbing speed, liquid permeation, gel stability and the like), as specifications, depending on use objects (for example, disposable diapers, sanitary napkins and the like). However, in the particulate water-absorbing agent to be consumed in a large amount, property stabilization in continuous production thereof is difficult, and a small variation of property may incur property decrease of a final product (for example, disposable diapers and the like) or consumer claims, and stable property control of has been a big problem. In order to solve such a problem of property stabilization, Patent Document 3 has disclosed a method for removing the water-absorbing resin having property out of the range of the upper and lower limits, and mixing them again. Patent Document 4 has disclosed technology of using a plurality of hoppers in an intermediate step. In addition, Patent Document 5 has disclosed a method for performing polymerization in two lines, and the latter half in one line. Many proposals have been provided on technology for enhancement and stabilization of property of the particulate water-absorbing agent, by changing or furnishing a new intermediate production step, in Patent Documents 2, 3, 4, 5 and the like, however, there was sufficient room left to be improved.